1. Technical Field of the Invention
The present invention relates generally to photolithography, and more specifically, to dynamic photolithography systems.
2. Description of Related Art
Photolithography is a method of transferring a pattern or image onto a substrate. Some industrial uses of photolithography include the manufacture of products, such as flat panel displays, integrated circuits (ICs), IC packaging, planar lightwave circuits (photonics), printed circuit boards, flexible circuits/displays and wafer bumping. In its simplest form, a photolithography system operates by passing light through a mask or tool placed over a substrate having a photosensitive surface, such as a layer of photoresist. Typically, the mask is formed of a transparent material with a fixed opaque pattern inscribed on the surface. Due to the photosensitivity of the substrate surface, when placed in contact with the mask and exposed to light, the pattern inscribed on the mask is transferred onto the substrate surface.
Although the use of a mask provides for a high degree of precision and repeatability, traditional contact photolithography systems suffer from several limitations. One limitation is a manufacturing specification that restricts the size of the substrate to no greater than the size of the mask. For large substrates, it is difficult to produce and handle a mask of sufficient size to cover the entire substrate area. In addition, as technology has progressed, the size of features photolithographically transferred onto the substrate surface has decreased to 0.5 μm or smaller. To achieve such small feature sizes, more advanced systems use projection optics to separate the mask from the substrate, allowing for optical reduction in the transferred feature size. However, in order to transfer the pattern for the entire substrate using an optical reduction system, the size of the mask would necessarily be larger than the size of the substrate. Large masks are both unwieldy and expensive to produce. To overcome the problems associated with large masks, many photolithography systems use multiple masks that contain different portions of the total pattern. The pattern is stitched together on the substrate surface by altering the position of the substrate surface in relation to the mask.
However, the cost to design and embed a pattern on a mask is considerable, and therefore, creating a large number of masks may be cost-prohibitive. Likewise, in applications where frequent changes occur, creating a new mask each time a change occurs may not be cost effective. As a result, dynamic photolithography systems have developed that enable a manufacturer to dynamically change a mask pattern without requiring a new mask for each change. Dynamic photolithography systems commonly employ a spatial light modulator (SLM) to define a pattern that is imaged onto the substrate surface. SLMs are electrically controlled devices that include individually controllable light modulation elements that define pixels of an image in response to electrical signals.
Typically, at feature sizes of 0.5 μm or smaller, there are tens of millions of light modulation elements within an SLM that is not more than a few square centimeters in area. With the small SLM size, multiple exposures are generally required to image the entire area of the substrate. Since the image formed by the SLM is easily reconfigurable, it is a relatively simple process to divide the final image into sections, configure the SLM to transfer one of the image sections onto the appropriate area of the substrate surface, shift the relative position of the substrate and SLM and repeat the process for each image section until the entire image is transferred onto the substrate surface.
However, it is impracticable to assume that the SLM will be free from defects. Statistically, there will be at least a few of the tens of millions of light modulation elements of the SLM that are defective. As a result of the multiple imaging process, each defective light modulation element will produce numerous defects on the substrate surface. What is needed is a mechanism to mitigate the effect of defective light modulation elements.